1. Field of the Invention
The present invention relates to a photo mask having alignment marks and more particularly to a photomask having alignment marks for detecting undesirable location of stepper blades and undesirable mask pattern array on a wafer.
2. Description of the Prior Art
A conventional type of a photomask will be described, in referring to FIG.1.
FIG.1 shows an arrangement view of a photomask in company with stepper blades which consist of top stepper blade 3a, bottom stepper blade 3b, left stepper blade 3c, right stepper blade 3d.
Each stepper blade making a right angle moves left and right toward pattern area 1 and remains the necessary part, that is, pattern area 1, according to a shape of a photomask 12 and cuts off ultraviolet rays from the unnecessary part of photo mask.
However, when the stepper blades are inaccurately located because of input errors, a photo mask is more open or covered.
So, if a photo mask is more open, an unnecessary pattern and a unintended peripheral area of wafer is exposed and a product die of wafer is inferior if a photo mask is more covered, a necessary pair of pattern and wafer is not exposed and a product die of wafer is inferior
Furthermore, the prior art have one or more collateral difficulties detecting misalignment of stepper blades and the array of product dies on the wafer when the first pattern is formed,
Accordingly, the object of the present invention is to provide a photomask having alignment marks for detecting undesirable location of stepper blades and undesirable mask pattern array on a wafer.